(a) Field of the Invention
The present invention relates to an automatic transmission, and more particularly, to a shift control method for an automatic transmission which improves shift responsiveness of the same to enhance ride quality.
(b) Description of Related Art
An automatic transmission provides necessary gear ratios to adapt vehicle operation to a wide range of speeds and loads. It does this with a minimum amount of effort on the part of the driver. That is, automatic upshifts and downshifts are made convenient for the driver because a foot-operated clutch is not required to shift gears, because the vehicle can be brought to a stop without the use of a clutch, and without shifting the transmission into neutral.
A conventional automatic transmission for a vehicle comprises a torque converter provided between an engine and the transmission to act as the fluid link therebetween. The torque converter has an impeller, a turbine, and a stator. A gear train is connected to the torque converter to provide various forward speed ranges and reverse; a plurality of friction members such as disc clutches and one-way clutches which control gear action, are provided. A hydraulic control system is provided for controlling the operation of the friction members.
Although the automatic transmission is somewhat complicated in structure and manufacture of the same is difficult, because of its many advantages over the manual transmission (e.g., convenience to the driver, improved ride comfort, reduced fuel consumption, etc) automatic transmissions are widely used and manufacturers of vehicles continue to put forth much effort in their development.
One area of concentration in the development of automatic transmissions has been that of skip shifting. Namely, automatic transmissions are now able to undergo 3-1 closed-throttle downshifting, 4-2 and 3-1 full-throttle forced downshifting, 2-4 closed-throttle upshifting, and other such skip-shifting patterns.
Before such development, when quickly shifting from a fourth speed to a second speed, for example, shifting would occur first into the third speed, then before shifting into the third speed was completed, shifting was realized into the second speed. Another method was the direct shifting into the second speed after temporary shifting into a neutral N range. In either case, shift quality was poor.
A typical example of a prior art automatic transmission which solves the above problem by enabling skip shifting is U.S. Pat. No. 5,536,216. FIGS. 1 and 7 of U.S. Pat. No. 5,536,216, illustrate, respectively, a hydraulic circuit diagram of U.S. Pat. No. 5,536,216 and a hydraulic circuit diagram of the prior art automatic transmission in which skip shifting is not possible. Because plurality of valves are added to the hydraulic control system of U.S. Pat. No. 5,536,216, the hydraulic circuit diagram of the same is much more complicated than that of the prior art hydraulic control system in which skip shifting is not possible.
This results in a difficult manufacturing process and an increase in overall manufacturing costs.